Alteration of the individual purine and pyrimidine bases of DNA may result in cellular changes which lead to somatic mutation and carcinogenesis. This type of damage is distinguished from the types of damage which produce changes in the macromolecular structure of the DNA and which are easily recognized by enzyme systems or which are so extensive that lethality result. A base alteration is more likely to be mutagenic since it may be transcribed as a different base. The extent to which base alterations may occur in a cell as a result of ionizing radiation is unknown. This project involves the design of an assay procedure for measuring radiation induced damage to one of the bases in the cellular DNA, and further, to determine the extent to which the cell is able to repair such damage. Adenine is chosen as a marker for base damage because purines may be released from irradiated DNA by mild hydrolysis. Damaged purines may subsequently be isolated by chromatographic procedures. Cellular systems to be studied are chosen due to their ability to repair radiation damage. If repair of base damage can be determined, factors affecting the repair can be studied. This work is basic to the understanding and possible control of radiation induced carcinogenesis.